Evidence of Huge Difference between Ground- and Excited-State Curves: NUV-Excited, Red (Ba, Sr, Ca)7(SiO3)6Cl2:Eu2+ Having a Large Stokes Shift

Abstract

According to our previous experimental results, the comparison concerning the temperature dependence of emission half-width based on Henderson-Imbush equation between (Ca,Sr)₇(SiO₃)₆Cl₂:Eu²⁺ (CaSSC), having a large Stokes shift from NUV to yellow, and (Ba,Sr,Ca)₇(SiO₃)₆Cl₂:Eu²⁺ (BaSSC), having a further larger Stokes shift from NUV to red, revealed that the traditional theory “Stokes shift = 2 × S × ℏω or (2S – 1) × ℏω” (S, Huang–Rhys parameter; ℏω, quantized phonon energy; ω, angular frequency of vibration) does not hold. Here, we tried to judge whether the equality in the two curvatures of ground and excited states in the configurational coordinate model, the basis for the above equation, is precise or incorrect. In this paper, we performed a simulation by using the quantitative data of BaSSC. By using the data of excitation–emission spectra and the temperature dependence of emission intensity, we determined the activation energy for excited electrons exceeding the intersection of two ground- and excited-state curves (nonemissive process) in addition to excitation energy and emission energy. These values were used to pursue a relationship between two coupling constants in two harmonic oscillators at the ground state and excited state and the equilibrium position offset between the ground- and excited-state curves in Eu²⁺-anion. Although the equilibrium position offset did not converge, interestingly, the ratio of the two coupling constants in the ground and excited states converged and was able to be determined to be 0.23 ± 0.01. It was found that the coupling constant in the excited state does not match that in the ground state at all, being quite smaller, 1/4, or less. It was strongly considered that the excited-state curve is much looser than the ground-state curve in the case of Eu²⁺ phosphors having a large Stokes shift.